1. Field of the Invention
The invention generally relates to a method for the production of calcium sulfate alpha-hemihydrate and, more particularly, to the conversion of other non-hydrated or partially- to fully-hydrated forms of calcium sulfate to calcium sulfate alpha-hemihydrate.
2. Brief Description of Related Technology
Calcium sulfate hemihydrate, commonly referred to as xe2x80x9ccalcined gypsum,xe2x80x9d xe2x80x9cstucco,xe2x80x9d or xe2x80x9cplaster of Paris,xe2x80x9d is used in many applications, such as producing molded articles and wallboard for the construction industry. Calcium sulfate hemihydrate has a number of desirable physical properties including, but not limited to, its fire resistance, thermal and hydrometric dimensional stability, compressive strength, and neutral pH.
Typically, calcium sulfate hemihydrate is prepared by drying, grinding, and calcining natural gypsum rock (i.e., calcium sulfate dihydrate). Gypsum as a raw material is also a byproduct in the production of phosphoric, boric, and some organic acids, resulting from the reaction between their calcium salts and sulphuric acid, and is a secondary material in processing some minerals, production of pigments, and in desulfurization of flue gases from burning fossil fuels. The drying step of calcium sulfate hemihydrate manufacture includes passing crude gypsum rock through a rotary kiln to remove any free moisture present in the rock from rain or snow, for example. The dried rock then is passed through a roller mill (or impact mill types of pulverizers), wherein the rock is ground or comminuted to a desired fineness. The degree of comminution is determined by the ultimate use. The dried, fine-ground gypsum can be referred to as xe2x80x9cland plasterxe2x80x9d regardless of its intended use. The land plaster is used as feed to calcination processes for conversion to stucco. Land plaster typically has a degree of impurities such as clay and strontium sulfate, from about 5% by weight to about 20% by weight. Small amounts of calcium sulfate xcex2-hemihydrate and/or calcium sulfate anhydrite can also be present in land plaster.
The calcination (or dehydration) step in the manufacture of calcium sulfate hemihydrate is performed by heating the land plaster, and generally can be described by the following chemical equation which shows that heating calcium sulfate dihydrate yields calcium sulfate hemihydrate (stucco) and water vapor:
CaSO4.2H2O+heatxe2x86x92CaSO4.xc2xdH2O+1xc2xdH2O. 
This calcination process step is performed in a xe2x80x9ccalciner,xe2x80x9d of which there are several types known by those of skill in the art.
Upon further loss of water, calcium sulfate anhydrite is produced according to the following chemical equation:
CaSO4xc2x7xc2xdH2O+heatxe2x86x92CaSO4+xc2xdH2O. 
The presence of calcium sulfate anhydrite is generally not desired in a calcium sulfate xcex1-hemihydrate product.
Uncalcined calcium sulfate (i.e., land plaster) is the xe2x80x9cstablexe2x80x9d form of gypsum. However, calcined gypsum, or stucco, has the desirable property of being chemically reactive with water, and will xe2x80x9csetxe2x80x9d rather quickly when mixed with water. This setting reaction is actually a reversal of the above-described chemical reaction performed during the calcination step. The setting reaction proceeds according to the following chemical equation which shows that the calcium sulfate hemihydrate is rehydrated to its dihydrate state:
CaSO4.xc2xdH2O+1xc2xdH20xe2x86x92CaSO4. 2H2O+heat. 
The water requirement for addition to the stucco is enough to provide 1xc2xd moles of water per mole of calcium sulfate for the rehydration reaction plus sufficient water to create a slurry of workable consistency. The actual time required to complete the setting reaction generally depends upon the type or form of hemihydrate, the type of calciner, and the type of gypsum rock that are used to produce the gypsum, and can be controlled within certain limits by the use of additives such as retarders, set accelerators, and/or stabilizers, for example.
Calcium sulfate hemihydrate occurs in two forms, alpha type (referred to herein as calcium sulfate alpha-hemihydrate, calcium sulfate xcex1-hemihydrate, or simply xcex1-hemihydrate) and beta type (referred to herein as calcium sulfate beta-hemihydrate, calcium sulfate xcex2-hemihydrate, or xcex2-hemihydrate). Calcium sulfate xcex1-hemihydrate is generally characterized by needle-shaped crystals which have a lower water requirement, set faster (i.e., produce calcium sulfate dihydrate faster), and produce articles of higher strength. The formation of calcium sulfate xcex1-hemihydrate from calcium sulfate dihydrate can be confirmed by scanning electron micrographs (SEM), differential scanning calorimetry (DSC) and various other methods.
Various methods are known for producing calcium sulfate xcex1-hemihydrate of varying quality from calcium sulfate dihydrate. Calcium sulfate xcex2-hemihydrate can also be converted to calcium sulfate xcex1-hemihydrate by first forming calcium sulfate dihydrate.
One process of forming calcium sulfate xcex1-hemihydrate involves placing calcium sulfate dihydrate in an autoclave in the presence of saturated steam at elevated pressure over an extended period of time (e.g., one to three hours). See, e.g., U.S. Pat. No. 5,015,450 (May 14, 1991), the disclosure of which is hereby incorporated herein by reference. In another process, gypsum is added to an aqueous solution including a crystallization accelerator and heated over an extended period of time under increased pressure while keeping the slurry in an agitated state. See, e.g., U.S. Pat. No. 4,842,842 (Jun. 27, 1989) and U.S. Pat. No. 4,091,080 (May 23, 1978), the disclosures of which are hereby incorporated herein by reference.
In still another process, gypsum is suspended in an aqueous solution, at atmospheric pressure, containing a soluble inorganic salt such as magnesium sulfate, sodium chloride, or calcium chloride, an inorganic acid such as sulfuric acid, nitric acid, or phosphoric acid, or an alkali metal salt of an organic acid, and heated at a temperature between about 80xc2x0 C. and the boiling point of the solution. See, e.g., U.S. Pat. No. 4,091,080, Kostic-Pulek et al., xe2x80x9cDeveloping a Hydrothermal Technique for Production of Alpha-Hemihydrate Calcium-Sulphate from Flue Gas Gypsum,xe2x80x9d Ceramicsxe2x80x94Silikaty 40 (3) 99-102 (1996), and Zxc3xcrz et al., xe2x80x9cAutoclave-Free Formation of xcex1-Hemihydrate Gypsum,xe2x80x9d J. Am. Ceram. Soc.74(5) 1117-24 (1991).
All of these methods are typically batch operations wherein the resultant product is filtered from the solution, washed with hot water to remove the inorganic salt, acid, or other catalyst from the surface of the crystals, and then heated to dry surface moisture from the crystals. Alternatively, the filtered solid has been washed with anhydrous solvents such as ethanol and/or acetone to both dry the crystals and remove residual catalysts. The time-limiting step in the process typically is the step of forming calcium sulfate xcex1-hemihydrate.
The use of microwave energy to dry surface moisture from calcium sulfate xcex1-hemihydrate has been reported by Zxc3xcrz et al., xe2x80x9cAutoclave-Free Formation of xcex1-Hemihydrate Gypsum,xe2x80x9d J. Am. Ceram. Soc. 74(5) 1117-24 (1991). Unlike conventional heating, microwave heating is more efficient because the radiation is absorbed directly by water molecules and dissipated as heat through vibration of the water molecules. In addition, microwave heating can often more quickly and uniformly raise the temperature of a given sample of matter because the radiation penetrates through non-absorbing matter, and heating absorbing matter from the inside.
The use of microwave energy to dry surface moisture from gypsum (calcium sulfate dihydrate) has also been reported. See, e.g., Lindroth et al., xe2x80x9cMicrowave drying of flue gas desulfurized (by-product) gypsum,xe2x80x9d Intl. J. Surface Mining, Reclamation and Environ. 9 169-77 (1995) and Turk et al., xe2x80x9cThe Effect of Sorbed Water on the Determination of Phase Composition of CaSO4.H2O Systems by Various Methods,xe2x80x9d The Chemistry and Technology of Gypsum, ASTM STP 861, R. A. Kuntze, Ed., 48-56 (1984). Evans et al. in xe2x80x9cA Study of the Dielectric Properties of Gypsum and their Relation to Microwave Drying Behaviour,xe2x80x9d Proceedings of Microwave and High Frequency Heating Conference, St Johns College, Cambridge (1995), reported that the use of microwave energy to remove free moisture is feasible, but that if sufficient energy is supplied to the product and temperatures of about 120+ C. are obtained, the conversion to hemihydrate will initiate, and a runaway reaction forming the anhydrite will occur.
Accordingly, it would be desirable to provide a method for rapidly and efficiently producing calcium sulfate xcex1-hemihydrate from forms of calcium sulfate without formation of substantial calcium sulfate anhydrite.
It is an objective of the invention to overcome one or more of the problems described above.
Accordingly, one aspect of the invention is a method of producing calcium sulfate xcex1-hemihydrate including the steps of exposing a mixture including a calcium sulfate form selected from the group consisting of calcium sulfate dihydrate, calcium sulfate xcex2-hemihydrate, soluble calcium sulfate anhydrite, and mixtures thereof, water, and a crystallization catalyst, to microwave radiation to produce calcium sulfate xcex1-hemihydrate, and separating at least a portion of the calcium sulfate xcex1-hemihydrate to provide a solid including calcium sulfate xcex1-hemihydrate, and a remainder.
Another aspect of the invention is a method of producing calcium sulfate xcex1-hemihydrate, including the steps of exposing a mixture including a calcium sulfate form selected from the group consisting of calcium sulfate dihydrate, calcium sulfate xcex2-hemihydrate, soluble calcium sulfate anhydrite, and mixtures thereof, water, and a crystallization catalyst, to microwave radiation to produce calcium sulfate xcex1-hemihydrate, and isolating calcium sulfate xcex1-hemihydrate.
Still another aspect of the invention is a method of producing calcium sulfate xcex1-hemihydrate including the steps of forming a mixture including calcium sulfate dihydrate, water, and calcium chloride, wherein the calcium chloride is present at a concentration of at least about 2 molar, exposing the mixture to microwave radiation to produce calcium sulfate xcex1-hemihydrate, filtering at least a portion of the calcium sulfate xcex1-hemihydrate, and washing the filtered calcium sulfate xcex1-hemihydrate, for example with acetone or ethanol.
Further aspects and advantages of the invention may become apparent to those skilled in the art from a review of the following detailed description, taken in conjunction with the appended claims. While the invention is susceptible of embodiments in various forms, described hereinafter are specific embodiments of the invention with the understanding that the disclosure is illustrative, and is not intended to limit the invention to the specific embodiments described herein.